Sadly, not everything in life lives up to expectations. Labour governments, new Tory leaders and England's World Cup performances all come to mind. But a night at the opera at La Scala, such as I had this week, has to be one of the exceptions. If you get one of those currently fashionable umpteen-things-to-do-before-you-die books in your stocking this Christmas, check that it includes a visit to a La Scala premiere. If it doesn't, send it back. For without such an experience, I fear that you could be fated to depart this world unfulfilled.

I'll concede immediately that, where Italy is concerned, many otherwise sensible people from this country suffer dreadfully from Merchant Ivory disease. They have a genetic compulsion to see only what they want to see about Italy and Italians and wilfully to ignore the many aspects that challenge our romantic illusions. They see the nice churches without the nasty clericalism, the charm without the cruelty, the romance without the racism, the glories without the greed. The Berlusconi era has provided a bit of a reality check but memories are short and the allure is strong.

And I'll also concede that there is a lot wrong with opera in Italy. For one thing, no one has written a good new one for 80 years; nothing of lasting note since Turandot. The vocal tradition is being squandered too. The tenor Carlo Bergonzi once told me in an interview that there would be no 21st-century Italian tenors because the training system for singers has been allowed to collapse. A lot of Italian opera houses are also subsidy junkies, in the grip of corrupt managements and greedy unions, and only bother to open their doors a few dozen nights a year.

So I'm not saying that a visit to the opera in Italy is a visit to a world of lost content in which opera is woven into the whole fabric of Italian vernacular life. It isn't. I love the true story of Verdi and a visitor driving a horse and carriage along a lane near Busseto one afternoon and encountering a party of farm workers who doff their caps and spontaneously break into a chorus from I Lombardi. But it's not like that now. Modern Italy, like Bertolucci's Novecento, starts with the death of Verdi. We must stop ourselves sentimentalising about it.

And yet you can't be in Milan for long on the opening day of the Scala season without realising something peculiar to Italy is occurring. Each December 7, the Piazza della Scala becomes the nexus of Italian public life. Everyone is there, not just the president, the ministers, the mayor and the rich. The socialites and the broadcasters are out in force too, as well as the police. The December 7 rituals always involve demonstrations. One year, animal rights activists hurled blood and offal at fur-coated opera-goers. This year, there was a peaceful protest against plans to build a high-speed train line through the Val di Susa in the Alps and a campaign against cuts in the national arts budget.

There have been periodic efforts to tone down the La Scala "prima": pruning the guest list, cutting back on the goody bags, driving out some of the more obvious self-publicists. But it is still a plutocratic occasion, with audience arrivals covered Oscar-like on live television and cut-throat competition to get into the post-theatre gala dinner, where the artists enter to rounds of applause from diners.

Opera is a socially elite art form in Britain too, but all this is a world away from anything you would encounter here. Just as well, you may be tempted to think, and there are certainly two sides to the argument. But the Guardian letter writer who pointed out yesterday how little mainstream media coverage there was in Britain for the presentation of Harold Pinter's Nobel prize this week was right on the money. We don't take these things seriously. The Italians do.

Above all, though, they take the opera itself seriously. How could this not be so at La Scala, home of Bellini and Verdi, of Toscanini and de Sabata, of Pasta and Schipa? It was on this stage, 50 years ago, that Maria Callas, Cesare Valletti, Leonard Bernstein and Luchino Visconti combined to produce a performance of Bellini's La Sonnambula that is one of the most thrilling operatic recordings I know. Ghosts like that shape the La Scala experience even today. Going to any opera house in Italy is still a visit unlike any other. Standards vary, but Italian audiences retain a genuine sense of possession over what takes place there. They think of themselves as an integral part of the occasion, and they are right, because they are.

I have never actually heard members of the audience join in, but this is still said to happen sometimes. I have certainly heard Italian audiences loudly debating both the merits and demerits of a performance while it is still going on. When a singer cracks a note in a British house, an audience tends to be embarrassed but considerate and, above all, silent. In Italy, audiences are instantly demonstrative. There is much sharp intaking of breath, tut-tutting to neighbours and even vigorous discussion. Shouting and booing are still commonplace. Ten years ago, I even heard Pavarotti booed at La Scala for a slip, while in Pavarotti's home town of Modena, if you believe the aspiring tenor in Hemingway's A Farewell to Arms, audiences used to throw things at singers who displeased them.

There was nothing of this kind at Wednesday night's Scala first night of Mozart's Idomeneo. And such a thing would not have been deserved. But it might have happened. The British conductor Daniel Harding, dubbed the "ragazzino" - little lad - by the Italian press, told me he had been warned to expect it. As a foreigner presiding over Italian opera's greatest annual occasion, it was highly possible that Harding might be given the bird, especially by admirers of the autocratic former music director Riccardo Muti. Interestingly, it never happened. Instead, the Milanese audience gave Harding the biggest cheers of the night.

The simplest explanation of Harding's warm reception is also the obvious one. La Scala had launched its season with an exceptional production, very well performed. But there is a wider point too. The time was when only a mainstream Italian opera, conducted and sung by outstanding Italians, would suffice on such a night at what is still Italy's greatest opera house. Now, as the national tradition weakens, that is no longer an automatic possibility. There were only two Italian principals in Wednesday's cast. Italy is still Italy and La Scala is still La Scala. But both are changing. Little by little, for good and ill, they are becoming a bit more like everywhere else.